When prosthetic treatment involves the anterior teeth, there are two considerations that must be kept in mind: the functional and the esthetic factors that must be carefully designed to ensure a favourable outcome.
For the esthetic aspect of the design, many people often talk about the smile line, the golden proportion of teeth, the gingival contour and symmetry, the location of the incisal/gingival embrasures and gingival zeniths and much more. While they are very much important in designing the anterior crowns, bridges or veneers, I see more patients who have been dissatisfied with their anterior prosthetics not because the teeth do not look good but because of how their lips protrude as a result of the crowns, bridges and/or veneers.
As a prosthodontist, I often have to consider the lip contour especially for partial and complete edentulous cases. But even for people with no missing teeth, I often will evaluate the lip contour to see how the lip drapes over the anterior teeth. For some people, changing the anterior teeth can improve the lip contour when the teeth are lingually tipped or positioned. For others, changing the slightest contour of the teeth can change how the lip drapes over the teeth and may impact the lip support in a negative way.
To avoid a negative outcome, I always discuss the expectations of changing the front teeth relating to esthetics and phonetics. I also will most certainly fabricate a set of provisional restorations so the patients can test run how everything feels and looks in the mouth, including how the lip contour is. In this way, there is an opportunity to evaluate what works and what does not work for both the teeth and the lips. It is a reality check of what we can and cannot do. I find that spending time to educate the patient is critical to the success and one must factor that time in your treatment plan for the anterior cases